In the related art, a vehicle may be equipped with a suspension including air springs using compressed air. A vehicle may also be equipped with a vehicle height adjustment apparatus using air springs. There is a so-called closed type vehicle height adjustment apparatus. This type of vehicle height adjustment apparatus increases a vehicle height by supplying a working fluid (high-pressure air) stored in a pressure tank to the air springs that change the suspension state of each wheel. The vehicle height adjustment apparatus decreases the vehicle height by discharging the working fluid from the air springs back to the pressure tank. In a case where the vehicle height is adjusted, first, a target vehicle height is determined, and the working fluid is supplied to or discharged from the air springs in order for the vehicle height to reach the target vehicle height. In a case where the vehicle height reaches the target vehicle height, opening and closing valves to control the supply and discharge of the working fluid are closed, and vehicle height control is ended. In contrast, even if the opening and closing valves are closed, the working fluid may continue to flow through flow paths such that the air springs continue to be extended and contracted. That is, an actual vehicle height may overshoot the target vehicle height. There is proposed control technology for reducing the difference between the actual vehicle height and the target vehicle height caused by overshooting. For example, there is a vehicle height adjustment apparatus that estimates the amount of overshooting based on masses before and after adjustment of a vehicle height, and performs vehicle height control while taking the amount of overshooting into consideration. Japanese Patent No. 4517897 is an example of the related art.
In the related technology, a relationship between a mass and the amount of overshooting is taken into consideration during a design phase. Due to assembly errors occurring when air springs are assembled into a vehicle, or aging of the air springs over time, suspension performance when the air springs are designed may be different from that of when the air springs are used in actuality. For this reason, even if the amount of overshooting is acquired (estimated) based on the relationship between the mass and the amount of overshooting which is prepared in advance, the amount of overshooting may not correlate to actual behavior, and reliability may be decreased. As a result, an adjustment based on the estimated amount of overshooting may deviate from a target, and an ultimately reached vehicle height (actual vehicle height) may not be easily brought close to a target vehicle height (final target vehicle height) to which the vehicle height is desired to be adjusted in actuality.